1. Field of the Invention
The present invention relates to an image-forming apparatus, such as a mono-chrome/full-color electrophotographic copying machine, a printer, a facsimile and a composite machine thereof.
2. Description of the Related Art
In a full-color image-forming apparatus using an intermediate transfer system, a plurality of toner images formed on a latent-image supporting member with respectively different colors are once transferred and superposed on an intermediate transfer belt, and then transferred onto an image-recording medium such as paper at one time so that a color image is obtained. The transferring process from the latent-image supporting member to the intermediate transfer belt is referred to as “primary transfer”, and the transferring process from the intermediate transfer belt to the image-recording medium is referred to as “secondary transfer”. In particular, in the secondary transferring process, the secondary transfer roller is pressed onto the intermediate transfer belt with the toner image supported thereon, with an image-recording medium interposed therebetween, while a bias voltage is applied to the secondary transfer roller or the like to generate an electric field thereon. Thus, the toner image is secondarily transferred onto the image-recording medium.
In such a secondary transferring process, it has been proposed that in order to improve the image quality, one or more hard layers are formed on the surface of the intermediate transfer belt (JP-A No. 2007-212921 and JP-A No. 2007-17666).
However, in case of using the intermediate transfer belt with hard layers formed on the surface thereof, when a hard foreign matter is pressed onto the belt surface by the secondary transfer roller in the secondary transferring process, a crack occurs on the surface layer to cause a dent in the resin of a substrate. For this reason, problems, such as image defects and cleaning defects, tend to occur from the initial stage. Examples of the foreign matter mainly include those transferred from the outside by the image-recording medium or the like and filler particles contained in paper or the like.
In particular, in the case where endurance printing operations for a long time are carried out, many cracks occur on the surface of the intermediate transfer belt to cause an increase in the surface roughness of the surface of the intermediate transfer belt, with the result that image defects occur conspicuously. In the case where a conventional intermediate transfer belt made from only the resin substrate is used, since the belt surface is worn out, the surface is maintained smoothly even after the endurance printing operations. However, in the case of the intermediate transfer belt with the hard layer formed on the surface, since the surface is not effectively worn out, the surface roughness increases in a one-sided manner when endurance printing operations are carried out. As a result, the smoothness on the hard layer is lost, the transferring rate is lowered and lack in uniformity of density tends to occur conspicuously. Moreover, the toner particles are buried to cause conspicuous cleaning defects.
An object of the present invention is to provide an image-forming apparatus that makes it possible to suppress an increase in surface roughness of an intermediate transfer belt and consequently to suppress occurrence of image defects sufficiently for a long period of time, even when the intermediate transfer belt having a hard layer is used.